. The overall goal of the proposed research is to understand how the inflammatory response to gram-negative bacterial endotoxin (lipopolysaccharide, LPS) acts as a determinant of susceptibility to intoxication by xenobiotic agents. In concert with this goal, the major focus of this proposal is to understand the role of the neutrophil in LPS-induced amplification of hepatotoxic responses to xenobiotic agents. Evidence has emerged indicating that exposure to small, nontoxic amounts of LPS is an important determinant of the magnitude of response to several hepatotoxicants. Experiments in the first specific aim will determine in rats whether LPS influences the response to chemicals (e.g., aflatoxin B1, monocrotaline) that initiate hepatotoxicity by different mechanisms and produce different lesions. Neutrophils activated in tissues are likely to play an important role in the mechanism by which LPS amplifies hepatotoxic responses, and Aim 2 will address this hypothesis in vivo and in a co-culture system of neutrophils and hepatocytes (HCs). Previous studies indicate that activated neutrophils damage hepatic parenchymal cells (HCs) through the release of toxic proteases such as cathepsin G and elastase. Studies in Aim 3 will determine if cathepsin G and elastase appear in plasma before the onset of liver injury, if administration of anti-proteases prevents LPS potentiation of liver injury, if administration of the proteases into the portal vein of rats reproduces the effects of LPS in vivo, and if exposure of HCs in vitro to small concentrations of selected hepatotoxicants renders these cells more vulnerable to injury from cathepsin G and elastase. Tumor necrosis factor-alpha (TNF alpha) is released during LPS exposure and has pro-inflammatory effects on both neutrophils and hepatocytes. Aim 4 will explore the role of this important cytokine in the augmentation of chemical hepatotoxicity by LPS. In Aim 5, specific protein targets of neutrophil proteases on hepatocellular plasma membranes will be identified and their association with enhanced HC injury determined. Identification of such targets will provide important clues as to cellular mechanisms by which neutrophil proteases injure hepatocytes. These studies will reveal the role of neutrophils in LPS amplification of hepatotoxicity and mechanisms by which they act to increase susceptibility to hepatocellular damage from exposure to xenobiotic agents. The results will provide new knowledge about how LPS and the inflammatory events it generates may act as a determinant of the magnitude of response to toxicants.